Wet etching, which utilizes a solution of chemical etchant, is a common process in the fabrication of semiconductor devices. A typical solution of chemical etchant for polysilicon consists mainly of nitric acid (HNO3) as an oxidizer agent and hydrofluoric acid (HF) as a reducing agent in diluting agent of water. During the etching process, oxygen radicals of nitric acid react with silicon to form silicon oxide, which is then etched by hydrofluoric acid.
The polysilicon etch rate is generally determined by both the formation rate of silicon oxide through nitric acid and the removal rate of the silicon oxide by the hydrofluoric acid. High etch rates of about 300 to about 400 nm/min can be achieved with a volume ratio of 1 part of hydrofluoric acid in 3 parts of nitric acid in 5 parts of water.
Doped polysilicon is also utilized in semiconductor device fabrication process, for example, polysilicon can be doped with boron, phosphorous or arsenic to increase its conductivity. Further, in the device fabrication processes, etch selectivity between doped polysilicon and undoped polysilicon is also needed, requiring high etch rate for doped polysilicon film with minimum etch rate of undoped polysilicon, such as the silicon crystal substrate. The etch selectivity is not met by the typical polysilicon etchant, for example, the doped polysilicon usually cannot not be etched as precisely as necessary.
Etching solutions comprising hydrofluoric acid, nitric acid and acetic acid (HNA) has been shown to offer the required selectivity between highly doped polysilicon and lightly doped polysilicon. For example, an etching solution comprises 1 part of hydrofluoric acid, 3 parts of nitric acid, and 8 parts acetic acid by volume would achieve 0.7 to 3 micron/min for highly doped polysilicon while lightly doped polysilicon is not significantly etched.
Advanced semiconductor device fabrication requires smaller device geometry, and thus the need for uniform and precision etching in much lower etch rates. In a typical chemical etch process, the slow etch rate becomes critical to the successful fabrication of an intended device geometry.
Diluting the doped polysilicon HNA etchant with water or acetic acid can slow the etch rate, but can result in polysilicon residue. Thus there is a need for an etchant solution for doped silicon at low etch rates, such as less than 20 nm/min.